The patterning of semiconductor substrates requires the use of photomasks to project the pattern to be etched, either positive or negative, onto a photoresist. Because photomasks are repetitively imaged during their lifetime, a single defect can have a significant cumulative effect on yields. Defects may be in the form of residue or haze. Haze is typically the result of a chemical film or residue adsorbed to the photomask surface. These photomasks are becoming increasingly complex and expensive. Ideally, manufacturers should be able to clean photomasks multiple times to save costs. This is becoming increasingly difficult because of the materials used on the photomasks for the patterned layer and the fine features of the patterned layer. The photomasks are typically formed of chromium (Cr) or molybdenum silicide (MoSi) patterned layer formed over glass or quartz substrates. The cleaning of half-tone, or phase-shifting, masks presents greater challenges because the optical characteristics (such as transmittance and phase angle) must remain unchanged. The cleaning solution used must not etch the quartz or degrade the patterned layer of the photomask.
Additionally, the photomask needs to be cleaned regularly due to the build-up of a haze on the surface of the photomask under the pellicle during photolithography processing. The pellicle is an optically clear film that is suspended over the photomask by a frame that is glued to the surface of the photomask. To clean the photomask the pellicle and pellicle frame are removed. A residue of pellicle glue remains on the surface of the photomask. Thus, the cleaning solution used to clean the photomask not only needs to be extremely sensitive to the surface of the photomask such that the optical properties are not damaged, but the cleaning solution also needs to be able to remove the pellicle glue and the haze. If the pellicle glue is not removed and residues are left on the photomask this causes significant problems and the photomask cannot be reused.
The pellicle glue is typically a silicone adhesive. The removal of silicone residues from photomasks currently requires some kind of mechanical removal in addition to a chemical treatment. Heat is also typically required to remove the silicone pellicle glue. The mechanical removal may be followed by a high pressure rinse. Mechanical removal, high pressure, and heat are potentially very damaging to the patterned layer a photomask, and in particular to a patterned layer formed of a phase-shifting material such as MoSi. Additionally, multiple cleaning steps and rinses are required along with the mechanical removal. The multiple cleaning steps increase the likelihood that the photomask will be damaged.